The present invention is equally applicable to single-jet type meters and to multijet type meters. The following description is given solely for a single-jet type meter, but it will be understood that it is immediately transposable to the multijet case.
FIG. 1 is a diagram of a single-jet fluid meter. The flowing fluid whose transmitted volume is to be measured penetrates into a measurement chamber 10 via an inlet duct 12 and leaves the chamber via an outlet duct 14 after causing an impeller 16 to rotate. The number of revolutions performed by the impeller is proportional to the volume transmitted. A magnet 18 is secured to the impeller and drives a magnetic metering system (not shown) contained in a summing mechanism 20. The summing mechanism 20 is separated from the measurement chamber 10 in sealed manner by means of a plate 22. The face of the plate 22 that faces the measurement chamber 10 supports a pivot 24 perpendicular to the fluid flow direction. The pivot 24 defines the axis of rotation of the impeller. In equivalent manner, the pivot could alternatively be fixed to the opposite bottom wall 26 of the measurement chamber.
The impeller 16 includes a bearing 28 which constitutes a central component engaging the pivot 24 while the impeller is rotating.
At low flow rates in the example shown in FIG. 1, the impeller rests against the bottom wall by means of a stud 30 placed at the outside end of the bearing. As the flow rate increases, the impeller rises and the end of the central recess in the bearing comes into abutment against the tip of the pivot.
When the flowing fluid is loaded, e.g. when it is desired to meter irrigation water or heating water, prior art impeller meters quickly become jammed. The fine particles and the metal particles in suspension in the fluid, e.g. sand, mud, or clay, accumulate between the bearing and the pivot, thereby clogging the equipment.
Such clogging enhances particle accumulation and very quickly rotation of the impeller about the pivot is completely jammed.
It can also happen that "large" particles (i.e. having a diameter in the range 0.05 mm to more than 1 mm), entrained by turbulence in the fluid created between the bearing and the pivot, directly jam rotation. Furthermore, clogging causes the shaft to wear and thus reduces sensitivity of the meter.
French patent No. 2 056 913 proposes forming axial grooves in the recess of the bearing in which the pivot is located. These grooves are separated by ribs that are narrower than the grooves and that have rounded edges. The presence of the ribs makes it possible to reduce the rubbing area between the pivot and the bearing, thereby increasing sensitivity. Furthermore, the grooves also allow the fluid to flow axially and consequently make it possible to evacuate impurities that run the risk of damaging the bearing. As a result clogging is slowed down. However, in practice it is observed that a meter provided with such a device and used for metering irrigation water operates for no more than a few thousand cubic meters. Consequently, its lifetime before jamming lies in the range of a few weeks to a few months at most. Although the grooves enable the clogging phenomenon to be slowed down, it nevertheless persists. Quite quickly, the grooves become clogged and jamming then takes place.